The present invention relates to a screw joint for pipes, in particular sanitary and heating pipes of, for example, plastic material or a composite material of metal and plastics.
Screw joints for pipes are known per se and are generally made of metal. One example of a member used for a screw joint is a pipe connection fitting, the fitting body having an outer thread to connect the fitting with another element of the pipeline or a fitting. In this instant, the thread is meant to mechanically hold and connect the members of the screw joint. For sealing purposes, the thread of one member of the screw joint is fitted with hemp, a Teflon strip or similar sealing material, whereby a fluid-tight screwed joint is obtained after connection with the opposite member of the screw joint.
Recently, increased efforts have been made to produce screw joint members such as fittings, from plastic materials. Plastic fittings are advantageous over metal fittings in that they are considerably more resistant to corrosion. However, there are problems in sealing screw joints formed by two plastic members of a screw joint in threaded engagement. Other than metal, plastic material has a lower stress resistance so that upon screwing together plastic fitting members provided with sealing material, there is a danger of fissures occurring in the fittings members due to the increased radial mechanical tensions.
German Utility Model 74 09 347 discloses an outer connection thread with a seal for a fitting as defined in the precharacterizing part of claim 1. This fitting has a metal thread with a circumferential groove in its axial center portion, a plastic ring insert with an outer thread being located in the groove. The radial inside of the ring insert abuts the bottom of the groove. The outer thread of the ring insert projects beyond the outer thread of the fitting.
It is the object of the invention to provide a screw joint member, in particular of plastic material, for pipes that meets both the mechanical and the sealing requirements for screw joints so that the risk of damage to screwed fitting members is reduced.
According to the invention, the object is solved with a screw joint, in particular of plastic material, for pipes, preferably sanitary or heating pipes of, e.g., plastic material or a metal-plastics composite material, comprising
at least one screw joint member with a substantially cylindrical portion provided with a thread having at least one helically extending raised portion, in particular with a substantially triangular cross section, wherein
the at least one raised portion of the thread extending in the direction of the thread has a base section adjoining or located in the cylindrical portion and an end section radially adjoining the base section.
In the present invention, this screw joint is characterized in that, within at least a part of the course of the thread, at least the end section of the at least one raised portion is made of a material that is more elastic than the material of the cylindrical portion.
The screw joint of the present invention comprises at least one screw joint member formed with a substantially cylindrical portion provided with a thread (inner or outer thread). The thread may be seen as a raised portion of, in particular, triangular cross section formed helically along the cylindrical portion so that adjacent windings of raised portions are obtained and the entire thread extends along an imaginary axis (extension of the thread). The raised portion comprises a base section adjoining the cylindrical portion and an end section adjoining the former. The end section is made, in particular, of a plastic material that is more elastic than the material of the cylindrical portion of the at least one connection member. The thread does not necessarily have to be made of this material over its entire length in the end section of the raised portion; rather, it will suffice that only in a portion of the thread extension the end sections of the raised portion windings within this portion of the thread are made of this material.
When such a screw joint member (in particular of plastic material) is engaged with a matching screw joint member, this matching screw joint member may be made entirely of less elastic and, thus, harder material than the material the end section of the at least one raised portion of the screw joint member is made of in at least a portion of its extension. Since at least portions of the thread of the present screw joint member is made of a xe2x80x9chardxe2x80x9d plastic material and is thus able to accommodate the mechanical forces acting when the member is engaged, for example, with a metal counter screw joint member, the requirements regarding the mechanical stability of the screw joint are met by the present screw joint member. Due to the special design provided by the present invention in at least one other portion of the thread in which the more elastic material is used, the requirements regarding the sealing function of the screw joint are satisfied. Specifically, the thread portion including the end section of the raised portion of the thread that is made of more elastic material, is somewhat oversized so that the elastic material is reliably trapped in the windings with a certain pressure and fills the spaces entirely, assisting the fluid-tightness of the screw joint. In this manner, it becomes possible, for example, to make the present screw joint member from a plastic material, such as polysulphone, which is resistant to temperature, hydrolysis and chemicals, polysulphone being a rather hard and not very elastic material. The windings of a portion of the thread are covered with a tougher or more elastic plastic material, such as PEX or EPDM. This plastic material should also be a material resistant to temperature, chemicals and hydrolysis. The counter screw joint member may be made of metal, but it could just as well be of a solid plastic material resistant to temperature, chemicals and hydrolysis.
The two different material sections of the raised portion are in force engagement. This may take the form of a frictional engagement, a form fit or a material engagement. In any case, the connection of the different materials should be secured against rotation.
Further, it may be envisaged that the entire raised portion consists of the more elastic material in at least a part of the thread. Here, the base section of the rased portion does not coincide with he wall of the (hollow) cylindrical portion so that the end section of the raised portion projects radially inward (with an inner thread) or outward (with an outer thread) from the cylindrical portion.
Advantageously, the thread of the screw joint member of the present screw joint is slightly conical, the outer diameter being larger in the area with the more elastic material than in the remaining thread.
As is obvious from the above, the screw joint of the invention has a sealing effect due to the more elastic material provided. Therefore, this material may also be referred to as a xe2x80x9csealing materialxe2x80x9d.
It is possible to snake not only the end section of the raised portion defining the individual windings from the more elastic material, but also the base section. Finally, it may be envisaged to use the sealing material to fill the spaces between the raised portions provided with the sealing material so that the thread of the counter screw joint member xe2x80x9ccutsxe2x80x9d into this sealing material layer. When engaging two screw joint members, it is feasible to limit the course oft the, screwing by corresponding abutment surfaces on both members. In the present screw joint member it is advantageous to make the respective abutment surface from a material that is more elastic than the material of the cylindrical portion of the screw joint member. In particular, this material also is the sealing material that is also present in parts of the thread. The abutment surface is formed as a radially extending annular surface against which a corresponding abutment surface or end surface of the counter screw joint member presses when both members are fully engaged. Thereby, an additional sealing of the screw joint is obtained in this area.